The present invention relates to control switches, and more particularly to a control switch for automatically providing a safety function when the main contacts remain fused or stuck together in the "ON" position after the switch actuator has been moved to the "OFF" position.
During the operation of electrical equipment, occasions can arise in which the contacts of a control switch, e.g., a microswitch, become fused, welded or stuck together in their ON position. This fused condition can occur under heavy load when increased current is being drawn at the instant when the switch is turned OFF, with the result that there is arcing across the ON contacts occurs, and therefore fusing or welding of the contacts causing them to remain in their closed position, even though the switch operating means has been moved to OFF position.
For example, a control switch may be used to operate alternating current electrical equipment including a motor. If the switch actuator happens to be moved to the OFF position at an instant during a cycle of the alternating current when peak current is flowing in a heavily loaded motor, severe arcing with consequent welding can occur as the contacts start to part, due to the energy stored in the magnetic field of the motor which momentarily tends to maintain a large current flow through the contacts as they start to separate. Therefore, the contacts remain fused together in their ON condition, after the switch operating means has been turned OFF.
With a switch in this fused or "welded" condition, the circuit to the electrical equipment is still capable of energizing the equipment. Further, a switch in this welded condition is quite dangerous to the operator of the equipment, since the operator knows that steps have previously been taken to turn OFF the control switch, which normally opens (deactivates) the electrical circuit. Moreover, if the operator visually checks the switch actuating the mechanism, it is seen to be in its open circuit position. If the switch housing is visible and the operator visually checks, viewed from the exterior of the switch housing, the switch appears to be deactivated or turned OFF, because the visible switch actuating mechanism, e.g., an actuator or manually movable handle, is in its OFF position, However, the internal contacts, which are welded or stuck together, remain in their ON position.
The operator of the electrical equipment is lulled into a false sense of security when he/she looks at an operating control or at the switch and sees that the switch actuating mechanism is in its OFF position. Consequently, the operator incorrectly concludes that the circuit to the electrical equipment has been safely de-energized. The operator may then proceed to perform maintenance or cleaning of the equipment, assuming that the equipment cannot be activated. In the course of such service, the operator may accidentally engage a nearby operating switch. The circuit to the equipment is then completed and the equipment then proceeds to operate, with possible consequential damage to the equipment or injury to the operator. Moreover, if the equipment has been partially disassembled for maintenance, and then is activated, considerable damage may occur.
The terms "fused", "welded" or "stuck" are used herein interchangeably in a generic sense to mean that the main contacts of a switch are remaining together touching or joining each other in a current-conducting relationship, when these main contacts are intended to be separated from each other in a non-conducting relationship by moving the switch operating means to the OFF position.